This application claims the priority of Korean Patent Application No. 2003-83616 filed on Nov. 24, 2003 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an information storage medium, and more particularly, to a charge-dipole information storage medium having an improved storage capability by coupling an electric dipole obtained as a result of the ferroelectric effect to electric charges obtained via charge trapping.
2. Description of the Related Art
Conventional information storage media store information by making use of the charge trapping phenomenon or spontaneous polarization of a ferroelectric. More specifically, information is stored on an information storage medium using the charge trapping phenomenon by trapping electric charges into a thin film formed of an insulating or dielectric material such as SiO2, Si3N4, or ONO (Oxide/Nitride/Oxide). In general, free movement of electric charges in such a material is restricted. For instance, when electric charges are externally injected into such a dielectric material, the injected electric charges are trapped into trap sites of the dielectric material. This phenomenon is called charge trapping, and information can be stored by making use of charge trapping. However, when the trapped electric charges are electrically connected to the outside, they can easily escape from the dielectric material. Therefore, it is difficult to confine the injected electric charges into an information storage medium by charge trapping over a long period of time.
An information storage media which stores information by spontaneous polarization makes use of the characteristics of ferroelectrics. In general, by applying an electric field to a material a dipole moment is created, thus causing electric polarization. However, there is a particular material, called a ferroelectric, in which spontaneous polarization can occur without applying an electric field. A ferroelectric exhibits spontaneous polarization that can be reversed by applying an electric field. Lead zirconate titanate (PZT) is a representative ferroelectric expressed as Pb(Zr,Ti)O3. In an ABO3 perovskite crystal structure, a metal ion of Pb occupies the A-site of this structure, and Zr and Ti occupy the B-site in stoichiometric amounts. In general, a ferroelectric must exhibit a large polarization, fine tolerance for polarization fatigue, and high-speed polarization conversion.
As described above, information is written to or read from conventional information storage media, by making use of electric charge trapping or dipole characteristics.
FIGS. 1A and 1B illustrate conventional information storage media using a trapping media and a ferroelectric, respectively.
More specifically, FIG. 1A is a cross-sectional view of a conventional information storage medium using a trapping medium. Referring to FIG. 1A, a charge trapping layer 12, which traps electric charges therein, is formed on a substrate 11. The charge trapping layer 12 is formed of an insulating material such as SiO2, Si3N4, or ONO. When an electric potential is applied to the charge trapping layer 12, negative charges and positive charges are trapped and distributed as shown in FIG. 1A. In other words, the negative and positive charges form an electric field that is a long-range electric field. Accordingly, it is possible to detect electric charges without contacting the information storage medium with a probe used to write information on or read information from the information storage medium. However, even though the negative and positive charges form a long-range electric field, the information storage medium is disadvantageous in that its information retention time is very short.
FIG. 1B is a cross-sectional view of an information storage medium using a ferroelectric capable of generating electric dipoles. Referring to FIG. 1B, a ferroelectric layer 13, which generates electric dipoles, is formed on a substrate 11. In general, the ferroelectric layer 13 includes a PZT that is a ferroelectric. Application of an electric potential to the ferroelectric layer 13 causes electric polarization to occur as shown in FIG. 1B. Electric polarization causes formation of an electric field that has a shorter range than that of FIG. 1A due to charge trapping. Accordingly, a very short distance must be set between the information storage medium and a probe to write information to or read information from the information storage medium of FIG. 1B. Although the retention time of information stored in an information storage medium such as that shown in FIG. 1B is long, the probe must operate while contacting the information storage medium so as to measure the magnitude of an electric field generated by the electric dipoles. Consequently, the probe and the information recording media may become worn due to their mutual contact. Also, it is difficult to control problems caused by screening charges occurring on the ferroelectric layer 13 due to dust or moisture.